Fix pypa#12666 by pre-loading script wrapper code

src/pip/_vendor/distlib/scripts.py

@@ -49,6 +49,24 @@
     sys.exit(%(func)s())
 '''
 
+# Pre-fetch the contents of all executable wrapper stubs.
+# This is to address https://github.com/pypa/pip/issues/12666.
+# When updating pip, we rename the old pip in place before installing the
+# new version. If we try to fetch a wrapper *after* that rename, the finder
+# machinery will be confused as the package is no longer available at the
+# location where it was imported from. So we load everything into memory in
+# advance.
+
+# Issue 31: don't hardcode an absolute package name, but
+# determine it relative to the current package
+distlib_package = __name__.rsplit('.', 1)[0]
+
+WRAPPERS = {
+    r.name: r.bytes
+    for r in finder(distlib_package).iterator("")
+    if r.name.endswith(".exe")
+}
+
 
 def enquote_executable(executable):
     if ' ' in executable:
@@ -409,15 +427,11 @@ def _get_launcher(self, kind):
                 bits = '32'
             platform_suffix = '-arm' if get_platform() == 'win-arm64' else ''
             name = '%s%s%s.exe' % (kind, bits, platform_suffix)
-            # Issue 31: don't hardcode an absolute package name, but
-            # determine it relative to the current package
-            distlib_package = __name__.rsplit('.', 1)[0]
-            resource = finder(distlib_package).find(name)
-            if not resource:
+            if name not in WRAPPERS:
                 msg = ('Unable to find resource %s in package %s' %
                        (name, distlib_package))
                 raise ValueError(msg)
-            return resource.bytes
+            return WRAPPERS[name]
 
     # Public API follows
 

tests/functional/test_self_update.py

@@ -0,0 +1,22 @@
+# Check that pip can update itself correctly
+
+from typing import Any
+
+
+def test_self_update_editable(script: Any, pip_src: Any) -> None:
+    # Test that if we have an environment with pip installed in non-editable
+    # mode, that pip can safely update itself to an editable install.
+    # See https://github.com/pypa/pip/issues/12666 for details.
+
+    # Step 1. Install pip as non-editable. This is expected to succeed as
+    # the existing pip in the environment is installed in editable mode, so
+    # it only places a .pth file in the environment.
+    proc = script.pip("install", pip_src)
+    assert proc.returncode == 0
+    # Step 2. Using the pip we just installed, install pip *again*, but
+    # in editable mode. This could fail, as we'll need to uninstall the running
+    # pip in order to install the new copy, and uninstalling pip while it's
+    # running could fail. This test is specifically to ensure that doesn't
+    # happen...
+    proc = script.pip("install", "-e", pip_src)
+    assert proc.returncode == 0